"." known as HEAD will get you the most up to date ports/src/doc trees currently available.
I'd highly recommend using HEAD for ports and docs, but not for src.
If you do a build/install world after csupping HEAD from src, you'll end up with a 9.0-CURRENT system, which isn't reccommended unless you need to be tracking CURRENT.
However, HEAD will get you the latest docs (I don't even update my docs. I can check that crap online) and the latest ports tree with the most recent versions of all of your favourite programs.
EDIT: This is of course assuming your supfiles are separate, which mine are
ports-supfile and stable-upfile (named because I track 8.0-STABLE and just copied the example supfiles)